Sidewall containment of liquid metal with horizontal alternating magnetic fields

ABSTRACT

An apparatus for confining molten metal with a horizontal alternating magnetic field. In particular, this invention employs a magnet that can produce a horizontal alternating magnetic field to confine a molten metal at the edges of parallel horizontal rollers as a solid metal sheet is cast by counter-rotation of the rollers.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. W-31-109-ENG-38 between the U.S. Department of Energy and theUniversity of Chicago, operator of Argonne National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates generally to the casting of metal sheets and isparticularly directed to the vertical casting of metal sheets betweencounter rotating rollers.

Steel making occupies a central economic role and represents asignificant fraction of the energy consumption of many industrializednations. The bulk of steel making operations involves the production ofsteel plate and sheet. Present steel mill practice typically producesthin steel sheets by pouring liquid steel into a mold, whereupon theliquid steel solidifies upon contact with the cold mold surface. Thesolidified steel leaves the mold either as an ingot or as a continuousslab after it is cooled typically by water circulating within the moldwall during a solidification process. In either case, the solid steel isrelatively thick, e.g., 6 inches or greater, and must be subsequentlyprocessed to reduce the thickness to the desired value and to improvemetallurgical properties. The mold-formed steel is usually characterizedby a surface roughened by defects, such as cold folds, liquation, hottears and the like which result primarily from contact between the moldand the solidifying metallic shell. In addition, the steel ingot orsheet thus cast also frequently exhibits considerable alloy segregationin its surface zone due to the initial cooling of the metal surface fromthe direct application of a coolant. Subsequent fabrication steps, suchas rolling, extruding, forging and the like, usually require thescalping of the ingot or sheet prior to working to remove both thesurface defects as well as the alloy deficient zone adjacent to itssurface. These additional steps, of course, increase the complexity andexpense of steel production.

Steel sheet thickness reduction is accomplished by a rolling mill whichis very capital intensive and consumes large amounts of energy. Therolling process therefore contributes substantially to the cost of thesteel sheet. In a typical installation, a 10 inch thick steel slab mustbe manipulated by at least ten rolling machines to reduce its thickness.The rolling mill may extend as much as one-half mile and cost as much as$500 million.

Compared to current practice, a large reduction in steel sheet totalcost and in the energy required for its production could be achieved ifthe sheets could be cast in near net shape, i.e. in shape and sizeclosely approximating the final desired product. This would reduce therolling mill operation and would result in a large savings in energy.There are several technologies currently under development which attemptto achieve these advantages by forming the steel sheets in the castingprocess.

One approach under consideration by the steel industry to reduceprocessing involves roller casting of sheets of steel. This method wasoriginally conceived by H. Bessemer over 100 years ago as described inBritish patent nos. 11,317 (1847) and 49,053 (1857) and a paper to theIron and Steel Institute, U.K. (October 1891). This roller castingmethod produces steel sheets by pouring molten steel between counterrotating twin-rollers. The rollers are separated by a gap. Rotation ofthe rollers forces the molten metal through the gap between the rollers.Mechanical seals are necessary to contain the molten metal at the edgesof the rollers. The rollers are made from a metal with high thermalconductivity, such as copper or copper alloys, and water-cooled in orderto solidify the skin of the molten metal before it leaves the gapbetween the rollers. The metal leaves the rollers in the form of a stripor sheet. This sheet can be further cooled by water or other suitablemeans via jets. This method has the drawback that the mechanical sealsused to contain the molten metal at the roller edges are in physicalcontact with both the rotating rollers and molten metal and thereforesubject to water, leaking, clogging, freezing and large thermalgradients. Furthermore, contact between the mechanical seals and thesolidifying metal can cause irregularities along the edges of sheetscast in this manner thereby offsetting the advantages of the rollermethod.

Accordingly, it is an object of the present invention to provide animproved method and arrangement for casting thin metal sheets.

It is another object of the present invention to produce thin metalsheets which require little or no subsequent rolling after the sheet iscast.

Yet another object of the present invention is to reduce the cost andcomplexity of casting thin material sheets.

A still further object of the present invention is to produce thin metalsheets using less energy.

Still another object of the present invention is to produce a metalproduct having good metallurgical properties and surface characteristicsas it leaves the caster.

Another object of this invention is to provide for continuous rollercasting of metal sheets.

It is still another object of this invention to provide containment of apool of molten metal between twin-roller casters, without sidewalls thatmake physical contact with the rollers.

A further objective of this invention is to prevent a pool of moltenmetal from flowing out the ends of counter rotating rollers by means ofa shaped horizontal alternating magnetic field.

A further objective of this invention is to provide an electromagneticstopper or seal that is capable of preventing or regulating the flow ofa molten metal in a horizontal direction.

An additional object of the present invention is to electromagneticallycast metal sheets with a minimum of electromagnetic heating of themolten and solid metal.

Another object of the present invention is to provide a system andmethod which is particularly adapted for the continuous casting of thinsheets of steel.

SUMMARY OF THE INVENTION

The present invention provides for confinement of molten metal with ahorizontal alternating magnetic field. In particular, this inventionemploys a magnet that can produce a horizontal alternating magneticfield to confine a molten metal at the edges of parallel horizontalrollers as a solid metal sheet is cast by counter-rotation of therollers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross sectional front view of the present invention.

FIG. 1b is a sectional view of a segment of the roller in FIG. 1a.

FIG. 2 is a view along section line 2--2' of FIG. 1a.

FIG. 3 is a view along section line 3--3' of FIG. 1a.

FIG. 4 is a cross sectional view of the core as depicted along sectionline 4--4' or FIG. 2.

FIG. 5 is a perspective view of the magnet and coil of one embodiment ofthis invention.

FIG. 6 is a perspective view of another embodiment of the magnet andcoil of this invention.

FIG. 7 is a cross section of the yoke as depicted in FIG. 6.

FIG. 8 is a perspective view of another embodiment of the magnet core ofthis invention.

FIG. 9 is a front sectional vertical front view of another embodiment ofthis invention.

FIG. 10 is a vertical sectional front view of still another embodimentof the magnet of this invention and a sideview of the rollers.

FIG. 11 is a horizontal sectional view of another embodiment of thisinvention.

FIG. 12a is a front view of a portion of another embodiment of theroller rim of this invention.

FIG. 12b is a top view of the embodiment of the roller rim of thisinvention as depicted in FIG. 12a.

FIG. 13a is a view of a portion of a roller slowing another embodimentof the roller rim of this invention.

FIG. 13b is a sectional view along line 13b--13b' of FIG. 10.

FIG. 14 is a side view of another embodiment of this invention.

FIG. 15a is a side view of still another embodiment of this invention.

FIG. 15b is a horizontal view along line 15b--15b' of FIG. 15a.

DETAILED DESCRIPTION OF THE INVENTION

The present invention overcomes the problems of roller casting with anovel design which features electromagnetic containment of the liquidmetal at the roller edges in place of mechanical seals therebyovercoming the problems associated with mechanical seals. The presentinvention provides a shaped horizontal alternating magnetic field toconfine a pool of molten metal between the cylindrical surfaces of apair of rollers as the molten metal is cast into a thin vertical sheetby counter rotation of the rollers which force the molten metal betweenthem. The horizontal alternating magnetic field of the present inventioncan also be used to prevent or regulate the flow of molten metal fromweirs or orifices of other geometries. The pressure, p, exerted by themolten pool of metal consists essentially of ferrostatic pressure p_(h)and pressure p_(r) induced by the rollers via the solidifying metal tobe cast

    p=p.sub.h +p.sub.r.                                        (1)

The magnetic pressure, p_(m), exerted by the horizontal alternatingmagnetic field, B, must balance the pressure from the top of the metalpool to the region where the shell of the metal has solidifiedsufficiently thick to withstand the pressure. The magnetic pressure isgiven by

    p.sub.m =B.sup.2 /2 μ.sub.o                             (2)

where the constant μ_(o) is the permeability of free space,

The ferrostatic pressure p_(h) exerted by the molten pool of metalincreases linearly with increasing downward distance h from the surfaceof the pool

    p.sub.h =gρh                                           (3)

where ρ is the density of the metal and g is the acceleration ofgravity. The magnetic field required to contain the ferrostatic pressurecan be found by equating the magnetic and ferrostatic pressure,

    B=(2 μ.sub.o gρh).sup.1/2 =kh.sup.1/2.              (4)

For casting steel k is approximately 450 if h is measured in cm and B ingauss.

The roller induced pressure p_(r) depends on the properties of the metalbeing cast, the roller diameter and speed and the thickness of the metalstrip or sheet being cast, In case of steel sheets, it is estimated thatp_(r) can be many times larger than the hydrostatic pressure p_(h).

The frequency of the alternating magnetic field chosen is as low aspracticable consistent with the distance between the rollers and thedistance between the end of the rollers, typically between 39 Hz and16,000 Hz.

FIG. 1a depicts a cross sectional view of the roller casting arrangementof the present invention. A pair of rollers 10a and 10b (referred tocollectively as rollers 10) are parallel and adjacent to each other andlie in a horizontal plane so that a molten metal 12 can be containedbetween them above the point where the rollers are closest together.Rollers 10 are separated by a gap, d (shown in FIG. 2). Counter rotationof rollers 10a and 10b (in the direction shown by the arrows 11a and11b), operating with gravity, forces the molten metal 12 to flow throughthe gap d between the rollers 10 and out the bottom.

Magnetic poles 16a and 16b located on both sides of the gap d betweenrollers 10a and 10b generate an alternating magnetic field which exertsan electromagnetic inward force that prevents the molten liquid 12 fromflowing out the sides at the edges of the rollers 10a and 10b.Throughout this application references will be made to confinement atone end of a pair of rollers. It should be understood that confinementof molten metal between a pair of counter rotating rollers as providedby the present invention will be used at both ends of the pair ofrollers.

Rollers 10 include a cooling means to cool and thereby solidify themolten metal by conduction as it passes between rollers 10. Referring toFIG. 1b, the cooling means may comprise a plurality of circulatingwater-cooled channels 13 located inside the surface wall of the roller.Referring again to FIG. 1a, after emerging from rollers 10, the metalhas solidified into a sheet 18 having a thickness equal to the gap, d,between the rollers 10. Jets 22 located below the rollers further coolthe cast metal sheet by spraying a coolant (such as water or air) on it.The cast metal sheet is guided, supported and carried away from therollers by mechanical guides 23.

Referring to FIG. 2, there is depicted a horizontal sectional view ofthe invention along section line 2--2' of FIG. 1a. FIG. 2 depicts thearrangement of magnetic poles with respect to the rollers. Rollers 10aand 10b are separated by a gap, d, through which the metal being cast 18can pass. Magnet 24 is comprised of a yoke 26 and poles 16a and 16b.Coils 28a and 28b wind around the magnet. Coils 28a and 28b carry anelectric current supplied by an alternating current source therebymagnetizing the magnet 24 and inducing a magnetic field between poles16a and 16b. The major portions of magnetic poles 16a and 16b arelocated inside the outer edges 30a and 30b of the rollers. The magneticpoles 16a and 16b are stationary and radially separated from the rollers10a and 10b by a space clearance large enough to allow free rotation ofthe rollers 10. The poles 16 extend axially into the ends of the rollers10 a short distance.

The cylindrical surfaces of rollers 10 have a middle middle portion 32which comes in contact with the molten metal. The middle portions 32 areconstructed of a material which has high thermal conductivity so that acooling means, used in conjunction with the rollers, can remove heatfrom the molten metal thereby facilitating the casting process. In thepresent embodiment, the cooling means used in conjunction with therollers comprises water cooled channels 13 in the interior of rollers 10as shown in FIG. 1b. In this embodiment, the middle portions 32 ofrollers 10 are made of copper alloy.

The rollers 10 also have outer rims 34a and 34b which form extensions ofmiddle portions 32 of rollers 10. Rims 34 are located in the areabetween the magnetic poles 16. Poles 16 generate a magnetic field thatpenetrates through the rims 34 of rollers 10 in this embodiment.Therefore, for this embodiment rims 34 must be made of a materialsuitable for the transmission of a magnetic field. In this embodiment ofthe present invention, the rims are made of stainless steel.

The resistivity of stainless steel (approximately 75 micro-ohm-cm atroom temperature) matches reasonably the resistivity of molten steel(approximately 140 micro-ohm-cm); therefore, the horizontal magneticflux can penetrate both metals. Due to eddy currents in the moltenmetal, the field decays exponentially as axial distance, z, from theedge of the pool increases. Therefore, a magnet force F₁ at the pooledge is larger than the oppositely directed force F₂ further into thepool, as shown in FIG. 3, resulting in a net containing force F ##EQU1##As a result, the molten metal can be contained be ween the rollers.

Referring again to FIG. 2, the edges 30 of rollers 10 are curved andtapered on their interior portions to accomodate the magnetic poles 16.Likewise poles 16 generally conform in shape to the exterior portion ofthe rollers 10. Shield 33 encloses yoke 26 and portions of poles 16except for the pole ends. Yoke 26 may be made of a laminated core.Shield 33 encloses the core 26 without forming an electrically shortedturn as illustrated by FIG. 4. The shield 33 may be formed by twoU-channels 33a and 33b made copper sheets and insulated from each otherby at least one gap 35. Shield 33 should be made of a material with lowresistivity to prevent transmission of a magnetic field by means of eddycurrent shielding and thereby serve to reduce flux leakage, enhanceshaping the magnetic field and improve circuit efficiency. Shield 33 mayalso serve as a heat shield for the magnet and may be water cooled forthis purpose. A material with low resistivity and high thermalconductivity, such as copper or copper alloy, is ideal for use as shield33.

Referring to FIG. 3, there is depicted a horizontal cross section of thepresent invention as viewed along section line 3--3' of FIG. 1a. FIG. 3depicts a section between the rollers at a point displaced verticallyfrom the horizontal axes of the rollers 10. FIG. 3 shows containment ofthe molten metal 12 by the rollers 10 and the interaction of magneticfield, B, and eddy currents i. FIG. 3 depicts rollers 10 having middleportion 32 and rims 34. Also shown in FIG. 3 is the magnet 24 having ayoke 26, poles 16 coil 28 and shield 33.

FIG. 3 also depicts molten metal 12 retained between the ends of rollers10 by the magnet field, B (shown as the dashed lines), between poles 16.The magnetic field, B, causes eddy currents, i, in the molten metal,indicated by arrow heads out of the page and arrow tails into the page,and a resultant electromagnetic force, F, directed toward the interiorof the pool to contain the molten metal. The containment forces, F, aredue to the interaction of the horizontal field, B, with the eddycurrents, i, in the molten metal, induced in the molten metal by themagnetic field, B.

In the present invention, a number of different magnet and coilgeometries can be employed to adapt to particular requirements of thecasting process. FIG. 5 is an perspective view of the magnet 24 and coil28 as depicted in FIGS. 1-4. The magnet has laminated yoke 26 and poles16a and 16b. The poles 16 are arced in shape and may conform to theshape of the interior portions of rollers 10. The coil comprises a coilpair 28a and 28b which encircle laminated core portions 40a and 40b ofmagnet 24. Coils 28 are connected to an alternating current supply 36which provides an alternating current, I_(s), which energizes the magnet24. The pair of coils may be connected in series to the current sourceor in parallel depending upon design considerations. For simplicity, theeddy current shield around the magnet is not shown.

Another embodiment of the magnet and coil is depicted in FIG. 6. In thisembodiment, the magnet 42 has a square shaped core 44 connecting poles46a and 46b. Poles 46a and 46b in this embodiment have shaped pole faces48a and 48b but squared off backs 50 to conform to the square shape ofthe core 44. As illustrated by the cutaway view of pole 46b, aninsulated copper shield 51 encloses the core to reduce leakage flux. Agap 52 in the shield 51 prevents the shield from being a shorted turnaround the magnet core. Coil 60 encircles core 44 and shield 51. In thisembodiment, the coil 60 is a single layer coil instead of a coil pair asin the previous embodiment. Coil 60 is connected to a alternatingcurrent supply 36 which provides an alternating current, I_(s), whichenergizes the magnet 42. The leakage flux could be reduced further byalso enclosing the coil 60 with a copper shield 53a and 53b as depictedin FIG. 7. This additional shield 53a and 53b would reduce the crosssectional area available in the air space for the leakage flux aroundthe coil windings and thereby reduce such leakage flux. In still anotherembodiment, the inner shield 51 could be deleted and the core and coilassembly enclosed by only an outer shield 53.

FIG. 8 depicts another variation of the magnet used in the presentinvention. In this embodiment, magnet 54 has a generally truncatedtrapezoidal shaped core with rectangular flat arms 55 connecting the.trapezoidal yoke 56 to the poles 57a and 57b. Similar to the magnetdesign in FIG. 5, this magnet may have the advantage of being simpler toconstruct.

A further modification to the magnet is depicted in FIG. 9. In FIG. 9, amolten liquid 12 is being cast into a sheet 18 between rollers 10. As inthe previous embodiments, magnet poles 59a and 59b confine the moltenmetal at the edges of the rollers 10. In this embodiment, the magneticpoles 59 are adjustable in position. The poles 59a and 59b can beslanted and moved to be closer to or further away from the roller rims.This feature enables adjustment of the magnetic field. As depicted inFIG. 9, the upper parts of the poles 59 have been moved further awayfrom the roller rims as compared to the bottom part of the poles. Asshown by the dashed lines representing the magnetic field B in FIG. 9,with the top ends of poles 59 further apart, the magnetic field can bemade relatively stronger near the lower end and weaker at the higher endas compared to the pole configuration shown in FIG. 1a. Thisadjustability can be utilized for casting metal sheets of differentthickness where different forces of confinement may be necessary.

FIG. 10 shows still another variation of the magnet in the presentinvention. This variation offers the most flexibility of any of thedesigns shown so far. (FIG. 10 depicts just one magnet pole; it shouldbe understood that an identical pole would be positioned opposite thispole in the other roller.)

In FIG. 10, each magnet pole is divided into three discreet separatemagnetic elements 61a, 61b, and 61c. Each of these elements is anindependent magnet comprising cores 62, excitation coils 63, andeddy-current-shields 33, which enclose their respective coils and cores,except for an air gap which prevents the shields from becoming a shortedturn such as depicted in FIGS. 4 or 7. Magnetic element 61a contains theupper portion of the sidewall of the molten metal pool 12, element 61bcontains the center of the pool sidewall and element 61c contains thelower portion of the pool sidewall.

In this embodiment, each individual discreet magnetic element isindividually controlled and provided with individual currents, I_(sa),I_(sb), and I_(sc). These three magnetic elements may be energized froma single alternating current power source 64 or from three individualpower sources. With a single power source, two variable reactors wouldbe connected in series with the coils of two of the three magneticelements in order that the magnetic fields of the three magneticelements can be adjusted independently; the time constant (L/R) of thereactors is designed to be the same as the time constant of the magnetsin order that the flux generated by the three independent magnets is inphase. With three independent power sources, care must be taken that thethree sources have the correct phase relation. Because each element canbe individually adjusted there is provided a high degree ofadjustability for the total magnetic field as well. This adjustabilitycan be utilized to optimize operation under varying conditions, such aswith different sheet thicknesses, different molten metals or alloys,different temperature conditions, start-up and shut-down.

Feedback loops can utilize sensors 65 to monitor the position of theupper, middle and lower portions of the electromagnetically containedsidewall. Any deviation from a present position will produce an errorsignal which, after suitable amplification, will change the powersupplied to the respective magnetic elements in order to restore thepreset containment position of the respective sidewall portion. Thesesensors may take the from of discreet beams (rays) that are transmittedparallel to the sidewall from one side and detected by a receiver on theother side (the beam being interrupted when the sidewall moves closer tothe magnet). Alternately, the sensors may take the form of discretebeams that are transmitted normal to the sidewall and their reflectionfrom the surface of the sidewall being detected by a receiver and usedto determine the position of the sidewall. The sensors may take the formof variable capacitors where the monitored sidewall portion is oneelectrode of the capacitor and the other is a suitable electrode mounteda fixed distance and in parallel to the sidewall. In a still furtheralternative, the sensor may take the form of an impedance measurement ofthe magnet excitation which changes with the flux linkage between themagnet and the liquid metal of the respective sidewall portion.

A still further embodiment of the magnet design is depicted on FIG. 11.FIG. 11 depicts a horizontal sectional view of one end of one rollerpair. In this embodiment the pole assemblies 66a and 66b are hoop-shapedadd contained inside and attached to the rollers 10a and 30b behind rims34a and 34b, respectively. Accordingly, poles 66 will rotate with rims34 and rollers 10. Portion 68 of shield 69 is located between coresections 72a and 72b and close to the area where the casting takesplace. Poles 66a and 66b are circular and made of a ferromagneticmaterial. The coil 60 magnetizes yoke 70 and magnet arms 72a and 72b asin the previous embodiments. Eddy current shields 69 and 79 confine themagnetic flux to the yoke 70, magnet arms 72 and poles 66 (reducingleakage flux) as described earlier. Shields 69 and 79 may alsoincorporate heat shielding or cooling means to protect the coil or themagnet. Poles 66a and 66b though separated from magnet arms 72a and 72band rotating with rollers 10a and 10b, are magnetized by their closeproximity to arms 72a and 72b via relatively small gaps 74a and 74b.This embodiment has the advantage that the poles can be located as closetogether as physically possible, i.e. inside the rims. This designsimplifies the shape of the magnet yoke and permits the use of differentmagnet yokes and coils when the assembly of rollers 10 and poles 66 isused to cast different thicknesses of metal sheets. Casting sheets, i.e.0.4" thick would utilize a more powerful magnet assembly than casting0.04" thick metal sheets.

As described previously and shown in FIGS. 2, 3, and 11, the magneticfield penetrates through the outer rim portion of the rollers to confinethe molten metal. The present invention can also be practiced without aspecial rim portion provided a suitable material is used for therollers, such as a ceramic, which enables penetration by a magneticfield without generating eddy currents in the roller. However, in thepreferred embodiment, use of a rim portion on the rollers provides forshaping the magnetic field by establishing a well defined transitionfrom the area of a high magnetic flux near the edge of the roller to anarea of low magnetic flux further away from the roller's edge. Shapingthe magnetic field in this manner provides the advantages of bettercontrol of the magnetic field that contains the sidewall of the moltenpool of metal.

The present invention provides for shaping the magnetic field by using amaterial with a low resistivity, such as copper or copper alloy, for themain portion of the roller and a material with a higher resistivity forthe rim portion. The copper or copper alloy used for the main portionwill effectively prevent penetration of the magnetic field (except for asmall negligible skin layer on the surface) and will, at the same time,cool the molten metal efficiently causing it to solidify.

In the rim portion of the roller, it is essential to allow penetrationof the magnetic field to confine the sidewall of the molten metalbetween the two roller surfaces. The present invention includes severaldifferent embodiments of the rim portion designed to allow penetrationof the magnetic field. In one embodiment, this is accomplished byconnecting a rim made of a material with a much higher resistivity, suchas stainless steel, to the edges of the copper rollers. FIGS. 2, 3 and11 depict stainless steel rims 34 of this type. The stainless steel rimsmay be connected to the copper rollers by brazing, bolting or othersuitable methods. In addition to allowing penetration of the magneticfield, the stainless steel rims provide a smooth surface for the castingsurface in case the molten metal encroaches on the rim.

Another embodiment of the rim portion is depicted in FIGS. 12a and 12b.The roller 80 is made of a low resistivity material such as copper. Atthe edges around the circumference of the rollers are a plurality ofslots 82 all the way through the roller. The slots 82 extend a shortdistance, s, in the axial direction of the roller. The slots 82 permitthe magnetic flux in the edge portion, or rims of the rollers, definedby the slots. Although the slots can be left empty, it is preferred thatthe slots be filled with a material of relatively high resistivity suchas ceramic or stainless steel, which is insulated from the sides of theslots, or filled with a material of high magnetic permeability.Alternatively, the slot can be filled with laminations of highpermeability metal which are insulated from each other and from thesides of the slots. Leaving the slots empty would require that themagnetic field is shaped such that the molten metal is kept away fromthe slots at all times. Filling the slots provides a smooth surface incase the molten metal encroaches on part of the rim during the castingprocess. Slot dimensions can be determined based upon the application.An advantage of the slotted copper rim design is that it features a lowreluctance path for the magnetic flux, i.e. the slots, filled withhighly permeable material or with air, thereby enabling a high frequencyalternating magnetic field. For example, whereas the roller design withstainless steel rims can operate at relatively low frequencies, e.g. upto 500 Hz, the roller design with slotted rims can operate with a muchwider frequency range, e.g. up to at least 16 kHz.

Other embodiments of the rim portion are shown in FIGS. 13a and 13b.FIG. 13b is a horizontal cross section along line 13b--13b' of FIG. 10.The water-cooled rollers 10 are made of high thermal conductivitymaterial such as copper. At the edges and around the circumference ofthe rollers are one or more hoop-shaped extensions 91 of rollers 10.Arranged between these hoop-shaped extensions 91 are similar hoop-shapedmembers 92 made of copper. These hoops, 91 and 92, are insulated fromeach other and mounted to the rollers 10 with bolts 93. The bolts 93 areinsulated from the hoops to prevent electrical contact between theindividual hoops and between the hoops and the roller. The hoop-shapedextensions 91 serve the same purpose as the slots 82 in the previousembodiment, i.e. to transmit the magnetic field to the confinementregion. Extensions 91 can be made of similar materials as slots 82.Extensions 91 can be made of an insulating material, such as ceramic,having a high resistivity and relatively low permeability and,therefore, no eddy currents. Extensions 91 can be made of anon-magnetic, high resistively metal, such as stainless steel, whichalso has relatively low permeability, but has higher thermalconductivity than ceramic. Alternately, extensions 91 can be made of amagnetic material, such as silicon steel, which has high magneticpermeability and reasonable thermal conductivity. With a highpermeability material the hoop-shaped extensions themselves becomemagnetized. Thin insulated laminations of a ferromagnetic material couldbe used. With hoop-shaped extensions of stainless steel or ferromagneticmaterial, each hoop should be insulated from adjacent copper hoops. Thealternating flux emanating from the magnet pole penetrates the rollerthrough the hoops 91 and through the skin depth of the copper hoops 92.A portion of this flux induces eddy currents in the molten metal 12between the rollers. The interaction between the flux and the eddycurrents in the molten metal contains the sidewall of the molten metalpool between the rollers as described before. The thickness of thehoop-shaped extensions 91, the number of hoop-shaped extensions, thehoop-shaped extension material, and the magnet are designed to containthe sidewalls of the molten pool between the rollers. With thehoop-shaped extension made from highly permeable magnetic material, theelectromagnetic containment circuit is most efficient. In this case thereluctance of the magnetic circuit is mainly determined by thereluctance of the molten metal 12 and by the small air gap, 94, betweenhoops 91 and magnet pole 61c; all other designs have much larger airgaps and resultant larger leakage flux.

Another embodiment of this invention is shown in FIG. 14. Thisembodiment of the invention may be used where conditions are such thatthe edge of the cast metal sheet is not fully solidified by the time itexits from between the rollers, This condition may occur for a number ofreasons dictated by the casting process, such as the need for highmagnetic fields of relatively high frequency resulting in large eddycurrent heating of the edges of the metal being cast, insufficientcooling effect of the rollers near the edges, thick cast sheetdimensions, or a combination of these or other factors. FIG. 14 depictsthe rollers 10 and molten metal 12 as in previous embodiments. FIG. 14also shows poles 95a and 95b which extend below the center line ofrollers 10. This has the effect of also extending the magnetic fieldbelow the center line of the rollers thereby extending theelectromagnetic containment of the edges.

Wheel induced forces on the liquid edge of the metal sheet vanish whenthe sheet leaves the rollers. Only gravitational forces act on the stillmolten edges which may be cooled by gas flow or by water spray. Themagnetic forces between poles 95 decrease as the sheet moves furtherfrom the rollers; this is compatible with the solidifying edge as thesheet moves down. However, if the edge of the sheet is not quite solidnear the end of the magnetic field between poles 95, further confinementof the still molten edges of the sheet can be provided by supplementalmagnet with poles 96a and 96b which extend the magnetic field well belowrollers 10 until the metal sheet is sufficiently hard enough to besupported by mechanical guides 23.

Another embodiment of this invention is depicted in FIGS. 15a and 15b.This embodiment presents a combination of a magnetic and mechanicalmeans to contain a molten metal at the edges of a roller casting system.As mentioned above, the problem of using mechanical seals to contain amolten metal at the edges of counter-rotating casting rollers was thatthe mixture of the molten and solidifying metal in combination with therotation of the rollers would clog up around the mechanical seals. Asdescribed above, the present invention shows how a magnetic field can beused to contain the sidewalls of the molten metal. The presentembodiment uses both a mechanical seal and a magnetic field toadvantage. As in previous embodiments, rollers 10 and poles 16 contain amolten metal 12. The present embodiment also includes a mechanical dam100 positioned between poles 16a and 16b. Mechanical dam 100 is shapedso that it will contain the molten metal in that area where there islittle likelihood of clogging or deforming the cast sheet, i.e. awayfrom the solidifying effects of the rollers. As depicted in FIGS. 15aand 15b, mechanical dam 100 is spaced away from rollers 10. It is in theareas close to rollers 10 that the metal is solidifying and where thelikelihood of clogging is greatest. Magnetic confinement with the poles16 is used to confine the molten and solidifying metal in the gapsbetween the mechanical dam 100 and rollers 10. Mechanical dam 100 may bemade of a ferromagnetic material 101 so that it provides a lowreluctance path for the flux between the poles 16. The side of the damfacing the molten metal pool may be made of a layer of high temperatureceramic 102 covering a water cooled heat shield 103 in front of the highpermeability material which may be made from steel laminations or fromhigh temperature ferrite. This embodiment has the advantage of requiringless energy than the previous embodiments because the magnetic fieldalong the molten metal extends only over the gaps between the rollers 16and mechanical dam 100. Also, because the volume of the molten metalcontained by the magnetic field is smaller, there is less heating of themolten metal due to eddy currents. Various mechanical dam shapes can bedesigned for shaping flux density suitable for different castingrequirements.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of continuouscasting of sheets of metal comprised of the steps of:forcing a moltenmetal between counter rotating rollers; and confining the molten metalat the edges of said counter rotating rollers with the electromagneticforce produced by a horizontal alternating magnetic field which inducescurrents comprising substantially vertically-oriented loops in themolten metal, whereby a solid sheet of metal can be cast from saidrollers.
 2. The method of claim 1, further including the step of coolingsaid molten metal with a cooling means located inside said rollers. 3.The method of claim 2 in which said horizontal alternating magneticfield operates at a frequency between 30 Hz and 16,000 Hz.
 4. The methodof claim 2, further including the step of cooling the sheet of metalafter said sheet of metal passes from said rollers.
 5. The method ofclaim 2, further including the step of confining the metal sheet aftersaid sheet passes from said rollers with a second magnet located belowsaid rollers.
 6. A method of continuous casting of sheets of metal,comprising the steps of:providing a containment means having an openside for containing a molten metal, said containment means comprising apair of rollers substantially parallel and adjacent each other andsubstantially in a horizontal plane, wherein said rollers are separatedby a gap; providing a magnet capable of generating a substantiallyhorizontal alternating magnetic field, said magnet including magneticpoles located adjacent to the open side of said containment means;inducing eddy currents in a layer substantially at the surface of moltenmetal with said magnet, said eddy currents interacting with the magneticfield producing a force for containing the molten metal within thecontainment means; and counterrotating said rollers to force the moltenmetal through the gap.
 7. The method of continuous casting of sheets ofmetal as defined in claim 6, wherein said magnetic poles extend axiallyinto the ends of said pair of rollers.
 8. The method of continuouscasting of sheets of metal as defined in claim 6, wherein each of saidrollers include a middle portion and a rim portion, said middle portionof said rollers having a resistivity lower than said rim portions sothat transmission of the magnetic field by said magnet through saidmiddle portion is less than through said rim portion.
 9. The method ofcontinuous casting of sheets of metal as defined in claim 6, furtherincluding the step of providing a dam adjacent said rollers forcontaining at least a portion of the molten metal.
 10. A method ofcontinuous casting of sheets of metal, comprising the steps of:providinga containment means having an open side comprising a pair of rollers,said rollers being separated by a gap; providing a dam adjacent saidrollers; locating a magnet adjacent to said open side of saidcontainment means; generating a substantially horizontal alternatingmagnetic field between poles of said magnet and parallel to said openside of said containment means so that the molten metal can be confinedwithin said containment means by at least one of said magnetic field andsaid dam; and counter rotating said rollers to force the flow of amolten metal between said gap between said rollers.
 11. The method ofcontinuous casting of sheets of metal as defined in claim 10, whereineach of said rollers include a middle portion and a rim portion, saidmiddle portion of said rollers having a resistivity lower than said rimportions so that transmission of the magnetic field by said magnetthrough said middle portion is less than through said rim portion. 12.The method of continuous casting of sheets of metal as defined in claim10, wherein said magnet comprises magnetic poles located adjacent tosaid open side of said containment means and extending axially into theends of said rollers, a core connecting said magnetic poles, and a coilencircling said core, said coil capable of being responsive to a currentsource.
 13. The method of continuous casting of sheets of metal asdefined in claim 10, and further including roller cooling means forcooling the surfaces of said rollers whereby molten metal coming incontact with said rollers will tend to solidify.
 14. The method ofcontinuous casting of sheets of metal as defined in claim 10, whereinsaid dam is located between said magnetic poles and separated from saidrollers;whereby said dam in cooperation with a magnetic field betweensaid magnetic poles can confine said molten metal between said rollers.15. The method of continuous casting of sheets of metal as defined inclaim 10, wherein said dam comprises a ferromagnetic material.
 16. Themethod of continuous casting of sheets of metal as defined in claim 10,including a layer of high temperature ceramic attached to said dam on atleast one side of said dam on which said molten metal can be retained.17. The method of continuous casting of sheets of metal as defined inclaim 15, including a liquid-cooled heat shield located between said damand said layer of high temperature ceramic.